1. Field of the Invention
The present invention relates to an electrical capacitance proximity sensor which detects an approaching object by changes in capacitance. Priority is claimed on Japanese Patent Application No. 2004-145502, filed May 14, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, an electrical capacitance proximity sensor is known (for example, Japanese Unexamined Patent Application, First Publication No. 2000-48694 (Paragraph 0006, 0012, FIG. 1, FIG. 2)), where a detector electrode and an earth electrode are formed on one face (detection face) of an insulating substrate, and a shield electrode is formed on an other face (back face) such that it covers a formation area of the detector electrode and the earth electrode, thereby concentrating electric lines of force on the detection face, so that the effect of an object on the side faces and the back face of the detection face are eliminated.
Such an electrical capacitance proximity sensor is furnished with a detector circuit which includes, an oscillation circuit which oscillates at a frequency based on the capacitance between the detector electrode and the earth electrode, and a judgment circuit which determines the oscillation frequency of the oscillation circuit, and the detector circuit detects changes in capacitance resulting from an object approaching the detector electrode and the earth electrode to thereby determine the presence of an object and distance.
In the above-mentioned conventional electrical capacitance proximity sensor, in anticipation of a shielding effect, the earth electrode is formed in a shape such that it surrounds the detector electrode. However, from experiments performed by the inventor, it has become clear that detection sensitivity is high near the detector electrode, and not near the earth electrode.